I had this problem on a my new phone. Found the solution on a T-Mobile forum but it was difficult to find. This post is to link back to the correct solution so it’s easier to find and to save the correct solution:

5. On the “APNs” menu – you should see two options: “T-mobile US” and “Tmobile MMS”. There should be a green dot next to “T-Mobile US”. Select that “T-Mobile US” option (NOT the Tmobile MMS).

6. This will open an “edit access point” menu, and you will need to make the following edits:

First – click on the MMSC option and add the following text: mms.msg.eng.t-mobile.com/mms/wapenc

Second – click on the “APN type” option. It will probably say something like: “default,supl,hipri,fota” (without the quotation marks). You will need to tweak the end of it so the whole thing instead reads “default,supl,hipri,fota,mms” (without the quotation marks). As indicated here – there are no spaces between these commas.

Third – after making these two adjustments to your APN – make sure you click on the menu dots (three vertical dots) at the top right hand part of the “edit access point” menu. This will open up a drop down menu – and then you should select “Save”. If you don’t do this – the settings will not take effect.